The present invention relates generally to concrete casting methods and apparatus, and particularly to methods and apparatus for casting elongated prestressed concrete structures, e.g., utility poles. More specifically, the invention concerns the casting of prestressed concrete utility poles of octagonal cross-section.
Concrete casting of utility poles, e.g., poles used for supporting lighting fixtures and/or utility lines, is known. Such elongated structures have been cast in various cross-sectional shapes, e.g., circular, rectangular and octagonal. In a conventional process, concrete slurry is poured into a mold having the desired shape and is allowed to cure before removal of the mold from the casting (or removal of the casting from the mold). Typically, the mold contains reinforcement elements, e.g., rebar running longitudinally within the mold, that become part of the cast product and impart additional tensile strength to the cast concrete (which by itself has high compressive strength but very low tensile strength).
The assignee of the present application has, for more than a year, commercially produced rebar reinforced (non-prestressed) octagonal concrete lighting poles employing a clamshell-type mold form. In this process, the clamshell-type mold form is closed onto an elongated rail (i.e., pallet) supported on a pair of light-duty saw horse-like supports formed from lengths of angle iron. An octagonally shaped mold cavity is formed with the pallet top surface forming a lower surface of the resultant mold; the clamshell-type mold form forms six additional mold surfaces and an open top along which the eighth surface of the casting is formed. Concrete is poured and allowed to cure. Once the concrete has firmed-up, hinged halves of the clamshell-type mold form may be opened to permit lateral removal of the mold form from the casting. The mold form can be reused while the casting continues to cure on the pallet. Once curing is substantially complete, the casting may be removed from the pallet for finishing operations, storage, transportation, etc.
Cast concrete structures with substantially increased tensile strength can be obtained through known concrete prestressing techniques. Generally, in such known techniques, concrete is poured around high strength steel wires, cables or rods which are kept under considerable tension until the concrete has substantially completely set. The wires are then cut, and compressive forces are thereby imparted to the concrete through the bond between the steel and concrete. Additional tensile strength in the cast product results from the fact that when the structure receives a load, the compression imparted to the concrete by the prestressing elements is relieved on that portion that would otherwise be put into in tension by the load. In order to assure a strong bond between the tensioned steel wires and the concrete (which is required to avoid slippage), it is necessary to permit the casting to substantially completely cure before the tensioning elements are cut or otherwise disconnected from the tensioning fixture.
For certain high load applications, such as utility and lighting poles to be used in regions susceptible to high winds, e.g., hurricanes, the substantially greater structural strength afforded by prestressed concrete is highly desirable. As compared with conventional reinforced concrete casting operations, however, known industry techniques for casting prestressed concrete poles are labor and time intensive, and require additional materials (e.g., stressing elements) and costly casting (and tensioning) apparatus.
Pour casting into an open-top mold incorporating prestressing elements has been used to form prestressed concrete structures. As noted above, however, in such processes it has been necessary for the casting to remain in the mold form until the casting is substantially cured, in order to avoid slippage of the tensioned prestressing elements within the concrete. This may take between 16 and 20 hours. With the mold form occupied for this lengthy period of time, production rates per mold form are necessarily very low. To achieve higher production rates, it is necessary to employ additional mold forms (and associated tensioning apparatus), at concomitantly greater expense. Additionally, with apparatus known in the industry, castings having a cross-sectional dimension that reaches a maximum between opposed sides of the casting, e.g., octagonal poles, cannot easily be removed laterally from a mold cavity. Rather, removal of this type of casting from its mold cavity requires either an involved disassembly of the mold form, or an endwise removal operation, i.e., a longitudinal extraction of the casting from the mold form. In order to permit an endwise removal operation, an end wall of the mold form must be disassembled and removed. If the ends of the mold form are reinforced and specially configured to serve also as pretensioning headers, such a removal operation can to be difficult.
Centrifugal (spin) casting can be carried out to cast poles within a mold including tensioned prestressing elements. Such apparatus tend to be very costly, however.
The following patents teach particular apparatus and methods for casting elongated prestressed concrete products utilizing a mold form positioned between, or incorporating therein, headers of a wire pretensioning fixture:
COLLIER U.S. Pat. No. 832,594
DEIGAARD U.S. Pat. No. 3,269,494
CAZENAVE et al. U.S. Pat. No. 4,758,393
COLLIER discloses a mold forming a pair of opposed cavities of L-shaped cross-section, along which wires pretensioned between end brackets extend. Once the casting is set, the tensioned wires are severed at their ends, and the castings (angle posts) are removed from the mold box.
DEIGAARD discloses a mold form of rectangular cross-section intended for casting concrete poles having a longitudinal opening therethrough. The mold form is positioned within a wire pretensioning apparatus including an elongated very heavy base, which is preferably made of concrete and partially embedded in the ground. Pretensioning wires are extended between a pair of headers. The mold form itself is a multi-part open top structure. Once the concrete has hardened to such a point that engagement of the concrete with the tensioned cables will prevent any movement of the cables within the concrete, the cables are cut and the side plates of the mold are dismantled and removed.
CAZENAVE et al. disclose a mold form (xe2x80x9cimpressionxe2x80x9d) separable from a wire tensioning frame for use in making beams of prestressed concrete. Separability of the impression from the tensioning frame permits removal of the impression from the casting (and tensioning frame) for reuse in casting another beam while the first beam begins to dry. In the disclosed process, the tensioning frame is placed on the impression. Concrete is cast into the mold constituted by the combination of the frame and the impression. The concrete and the mold is vibrated and xe2x80x9crammed.xe2x80x9d Then, the impression is flipped over together with the tensioning frame, and the impression is removed from the partially cured casting for reuse. The mold form nests within the tensioning frame; the frame comprises sides and ends but no central floor.
In view of the foregoing, it is a principal object of the present invention to provide improved apparatus and methods for casting prestressed concrete products, particularly prestressed concrete utility poles and like elongated concrete structures.
It is a more specific object of the invention to provide apparatus and methods for casting elongated prestressed concrete structures, which improve production efficiency by permitting reuse of a mold form while a first casting is left undisturbed to continue to cure in a pretensioning fixture that remains stationary.
It is yet another object of the invention to reduce equipment costs by enabling a prestressed concrete pole casting operation to be carried out using clamshell-type mold forms of the same general type previously used for casting non-prestressed concrete poles.
It is a further object of the invention to provide a prestressing element pretensioning fixture of simple easy to use construction, and great strength/rigidity for withstanding considerable wire pretensioning forces, which fixture is usable together with a clamshell-type mold form in such a manner that the form is easily properly locatable on the pretensioning fixture for pouring and initial curing, and easily removable from the pretensioning fixture without disturbing the casting, to thereby permit the casting to continue to cure in the pretensioning fixture during reuse of the mold form.
One or more of the above-stated objects are achieved in accordance with the present invention, by a method of casting elongated prestressed concrete products. A clamshell-type mold form, including two hinged mold halves, is positioned on a first prestressing element pretensioning fixture. The mold halves are closed together over a set of prestressing elements pretensioned in the pretensioning fixture, to form a mold cavity with the prestressing elements extending therealong. Concrete slurry is dispensed into the mold cavity. The concrete slurry is permitted to cure, to thereby form a concrete casting. The mold halves are opened and the mold form is removed from the casting and the pretensioning fixture, after the casting has partially cured. The casting remains on the pretensioning fixture, and the pretensioning fixture remains stationary, during removal of the mold form. The casting is permitted to continue to cure on the pretensioning fixture after removal of the mold form, at least to such point that engagement of the concrete with the pretensioned prestressing elements will prevent movement of the prestressing elements within the concrete. Thereafter, the prestressing elements are released from the pretensioning fixture.
In a second aspect, the invention resides in a production system for carrying out the method described above. The system includes multiple pretensioning fixtures, a clamshell-type mold form, and an overhead conveyor for transporting the mold form as a unit from one of the pretensioning fixtures to another.
In a third aspect, the invention is embodied in a wire pretensioning fixture for use in casting elongated prestressed concrete structures. The fixture comprises a pair of elongated I-beams joined together in side-by-side relation to form a mold form-supporting base. A pair of upstanding headers are provided, one secured at each end of the base. At least one of the headers includes an anchor plate to which ends of tensioned prestressing elements may be secured. The anchor plate is secured between a pair of generally L-shaped side plates having base portions thereof fitted and secured within spaces defined between respective pairs of upper and lower I-beam flanges.
In a fourth aspect, the invention is embodied in an apparatus for casting elongated prestressed concrete products. A pretensioning fixture includes a pair of spaced headers between which prestressing elements may be pretensioned. A clamshell-type mold form including two hinged mold halves is removably positionable on the pretensioning fixture to thereby form a mold cavity along which prestressing elements pretensioned between the headers may extend.
The above and other objects, features and advantages of the present invention will be readily apparent and fully understood from the following detailed description of preferred embodiments, taken in connection with the appended drawings.